None.
Not Applicable.
This invention relates in general to antifriction bearings and, more particularly, to a bearing mounting in which the setting of a bearing may be altered during the operation of the bearing.
Machine tool spindles must operate with full rigidityxe2x80x94rigidity in the sense that the axis of rotation for the spindle remains precisely fixed with respect to the housing in which the spindle rotates. A variety of bearing arrangements exist for supporting machine tool spindles in housings, and each employs several antifriction bearings of one type or another. The typical antifriction bearing, irrespective of its type, has an outer race located within the housing, an inner race fitted around the spindle, and rolling elements located between raceways on the two races. In order to achieve spindle rigidity, no clearance should exist between the outer race and the housing, nor should any clearance exist between the inner race and the spindle. Furthermore, the bearing should operate without internal clearances between the rolling elements and the raceways. In short, the bearing should operate under a preload. Usually, preloaded bearings operate in pairs or other multiple arrangements, so that one of the bearings may be adjusted against another of the bearings.
For example, a spindle may be supported on two single row tapered roller bearings mounted in opposition. Both carry radial loads, while one takes thrust loading in one direction and the other takes thrust loads in the opposite direction. Usually, one of the inner races, typically called a cone, is advanced axially toward the other until the bearings are in preload. This type of mounting arrangement will transfer heavy radial and thrust loads, but the spindle can operate only a moderate speeds. Angular contact ball bearings, like tapered roller bearings, have the capacity to carry both radial and thrust loads, and while they can tolerate higher speeds, they do not have the load-carrying capacity of tapered roller bearings. For this reason many high speed spindles rotate on multiple angular contact ball bearings, some oriented to take thrust loads directed inwardly from the nose and the remainder oriented to take thrust loads in the opposite direction. The presence of multiple bearings in the mounting renders the procedure for setting the bearings extremely tedious and difficult.
Even when a machine tool mounting is assembled with the correct amount of preload in its spindle bearings, this provides no assurance that the preload will remain or that it will not change. In operation, the bearings generate heat, and this heat will cause the spindle and the housing to expand at different rates. This in turn disturbs and indeed changes the setting in the bearings. An increase in preload may damage the bearings, whereas a decrease may eliminate the preload altogether and produce internal clearances. Neither is desirable in a machine tool.
Apart from that, machine tool spindlesxe2x80x94or for that matter any rotating componentxe2x80x94should not operate at or near its resonant frequency. But machining operations will set up self-excited vibrations which in their more violent form are referred to as chatter. To avoid chatter machine tool operators can alter the spindle speed, feed rate or depth of cutxe2x80x94and sometimes these conditions need to be altered a considerable amount to the detriment of the machining operations.
To be sure, efforts have been undertaken to avoid the forgoing problems with specialized bearings. For example, The Timken Company developed a spindle bearing arrangement which relies on two tapered roller bearings mounted in opposition (FIG. 1A). One of the bearings is essentially a single row tapered roller bearing. The other has a floating rib ring which is backed by a hydraulic fluid and positions the rollers axially between the two raceways for the bearing. The pressure of the fluid determines the preload in the bearings and the stiffness in the spindle. It further provides automatic compensation for thermal expansion and contraction since a uniform pressure in the fluid behind the rib ring will yield a constant preload irrespective of the axial position of the two raceways. Also, the operator, by controlling the stiffness of the spindle, can vary the resonant frequency of the spindle and thus avoid chatter. However, the bearing arrangement still relies on tapered roller bearings and is thus confined to moderate spindle speeds. Moreover, the two bearings can only operate under preload which is desirable for machine tool spindles, but perhaps not for other applications. U.S. Pat. No. 3,716,280 discloses the bearing arrangement.
Another specialized bearing arrangement for machine tool spindles utilizes a combination of cylindrical roller bearings and angular ball bearings (FIG. 1B). The former carry most of the radial loading, while the latter transmit the lesser thrust loads. But cylindrical roller bearings inherently have internal clearances, for without internal clearances such bearing could not be assembled. To eliminate the clearances, the inner races of the cylindrical roller bearings have tapered bores and at those bores fit over tapered surfaces on the spindle. When the inner races are forced axially over the tapers on the spindle, the tapers on the spindle expand the inner races and thus eliminate clearances between the cylindrical rollers and the raceways. But to achieve the correct setting, stops are needed to axially locate the inner races on the spindle, and they must be positioned with considerable precision. As a practical matter, this requires multiple assembly attempts and grinds along the stops or the bearing faces until the bearings acquire the proper preload.
Ideally, the bearings which support a machine tool spindle in a housing should be capable of undergoing adjustment during the operation of the bearing so as to operate continuously at a prescribed preload, this irrespective of temperature variations in the spindle the housing and the bearings themselves. The capacity to adjust preload also enables the bearings to alter the resonant frequency of the spindle, and thus easily compensate for chatter. What has been said with regard to machine tool spindles applies as well to some other applications where one component rotates relative to another on antifriction. Indeed, in some applications it is desirable to have end play and thus have the capacity to adjust between end play and preload.
The present invention resides in a mounting that includes two members, one of which rotates relative to the other on a bearing that includes a raceway carried by one of the members, another raceway on a race carried by the other member, and rolling elements arranged in a row between the two raceways. The member which carries the race has an actuator sleeve which at its ends is fitted tightly to the member but has an intervening region which flexes. The sleeve together with the member to which it is fitted define a chamber. The intervening region of the sleeve fits behind the bearing race. When the chamber is pressurized, the intervening region of the sleeve flexes and changes the configuration of the race against which it bears. This, in turn, changes the setting of the bearing.